1. Field of the Invention
The present invention relates to a radio frequency identification (RFID) reader. More particularly, the present invention relates to a circuit and a method for echo cancellation of an RFID reader.
2. Description of the Related Art
FIG. 1 is a schematic diagram showing a conventional RFID reader 101 and several RFID tags, for example three RFID tags 102-104. The reader 101 emits ultra high frequency (UHF) electromagnetic waves to enable the tags 102-104 so that the tags 102-104 can backscatter data to the reader 101. FIG. 2 is a schematic diagram showing the circuit of a conventional RFID reader 101. The RFID reader 101 includes an RF oscillator 201, a power splitter 202, a power amplifier (PA) 203, a modulator 204, a circulator 205, an antenna 206, a mixer 207, a demodulator 209, and a control block 208.
The RF oscillator 201 provides a carrier signal. The power splitter 202 relays the carrier signal to the PA 203 and the down converter mixer 207. The PA 203 amplifies the carrier signal. The control block 208 provides the data for an RFID tag as the modulating signal to the modulator 204. The modulator 204 performs a modulation operation with the amplified carrier signal and the modulating signal, and then outputs the modulated signal. The circulator 205 relays the modulated signal to the antenna 206 and the modulated signal is transmitted to the tag. Later, when the tag responds by transmitting data back to the reader 101, the antenna 206 receives the backscatter signal and the circulator 205 relays the signal to the mixer 207. The mixer 207 shifts the received signal into a low frequency band and then provides the shifted signal to the demodulator 209. This transmission signal from the tag is also modulated. The demodulator 209 performs the demodulation to retrieve the data sent by the tag. Finally, a digital signal processor (DSP) included in the control block 208 receives the retrieved data for further processing.
The circulator 205 has three ports, namely, P1-P3. When the reader 101 is receiving data from a tag, the circulator 205 relays the signal received at the antenna 206 from the port P2 to the port P3. At the same time, the carrier signal originating from the RF oscillator 201 is also relayed by the circulator 205 from the port P1 to the port P3. Consequently, the mixer 207 not only receives the signal sent by the tag, but also receives the carrier signal leaking from the port P1 to the port P3. Here the leakage carrier signal is also known as “echo”.
The leakage carrier signal is very undesirable because the difference of power level between the leakage carrier signal and the desired signal from the tag can be as large as 110 dB. Even though a typical circulator has signal isolation at the range of 30-40 dB from the port P1 to the port P3, then the power level difference can still be as large as 80 dB, as shown in FIG. 3. FIG. 3 is a schematic diagram showing the normalized power spectral density of the received signal before the mixer 207 at the port P3. The received signal includes the leakage carrier signal 302 and the desired signal 301. In this case, the leakage carrier signal 302 virtually overwhelms the desired signal 301, making it very difficult for the reader 101 to retrieve the desired signal 301 from the received signal. Ideally, the circulator 205 should have a signal isolation of 110 dB from the port P1 to the port P3, but such a requirement is impractical because no such circulator exists. As a result, the echo problem greatly reduces the sensitivity and the spurious-free dynamic range (SFDR) of the reader 101.